1. Field of the Invention
The present invention relates generally to a cryostat for adiabatically storing very low temperature liquid cryogens and, in particular, to a structure for supplying a liquid cryogen to the interior of the cryostat.
2. Description of the Prior Art
FIG. 1 is a diagrammatic, cross section of one example of the structure of a prior-art cryostat disclosed in Japanese Utility Model Laid-open No. 78111/1985. As shown, the prior-art cryostat comprises a vessel 100 including an inner wall 1 serving to define a tank for storing a very low temperature liquid cryogen 5 and an outer shell 2 for accommodating the inner wall 1. A cryogen supply pipe 3 serves to guide the very low temperature cryogen 5 which is supplied from a storage container 7 through a supply pipe 8 to the bottom of the inner wall 1. An outlet 4 is disposed for allowing discharge of the gases resulting from the evaporation of the very low temperature liquid cryogen 5. A superconducting coil 6 is housed in the space defined by the inner wall 1 in such a manner as to be submerged in the very low temperature liquid cryogen 5. The supply pipe 8 is held by a port pipe 9 at the top of the vessel 100 and extends into the pipe 9 toward the lower end thereof. The cryogen supply pipe 3 extends from the lower end of the port pipe 9 towards the lower portion of the inner wall 1. The very low temperature liquid cryogen 5 can thus be introduced through the cryogen supply pipe 3 into the bottom of the inner wall 1.
The following is a description of the operation of the aforesaid prior-art cryostat. The inner wall 1 and the outer shell 2 are combined with each other to form a kind of vacuum bottle, and thus the vacuum space defined therebetween provide heat insulation. The very low temperature liquid cryogen 5 is supplied from the storage container 7 into the interior of the inner wall 1 through the supply pipe 8, the port pipe 9 and the cryogen supply pipe 3. The very low temperature liquid cryogen 5 is thus accommodated in the space defined by the inner wall 1. The thus-accommodated liquid cryogen 5 is caused to evaporate under the influence of various thermal factors such as heat conducted from the exterior and a slight quantity of heat generated by the superconducting coil 6 disposed in the space defined by the inner wall 1. The gas resulting from such evaporation of the liquid cryogen 5 is discharged through the outlet 4.
The aforesaid conventional type of cryostat has a structure in which a room-temperature side is connected to a very-low-temperature side through the path formed by the supply pipe, the part pipe and the cryogen supply pipe. This structure may conduct unwanted heat from the exterior to the very low temperature liquid cryogen accommodated in the vessel, and this might correspondingly accelerate the evaporation rate of the accommodated liquid cryogen. This phenomenon may lead to the problem that the operating period of the superconducting coil per unit supply of the very low temperature liquid cryogen is shortened.